Device for loss-free cryogen cooling of a cryostat configuration

ABSTRACT

A cooling device ( 7 ) for re-liquefying cryogenic gases, comprising an outer jacket ( 8 ) which delimits a vacuum chamber ( 9 ), and a cryocooler cold head ( 10 ) installed therein, which has at least two cold stages ( 11, 12 ) and is at least partially surrounded by a radiation shield ( 13 ) is characterized in that at least two cold stages ( 11, 12 ) of the cold head ( 10 ) are separately individually connected in a heat-conducting manner to a heat-transferring device ( 14   a,    14   b ) which can be inserted into the neck or suspension tubes ( 3   a,    3   b ) of a cryostat ( 1 ) for keeping at least two different cryogenic liquids ( 18   a,    18   b ). The cooling device can be easily retrofitted into existing cryostat configurations, in particular, those containing superconducting magnets and without (or with minimum) adjustment to permit operation with no or little cryogen loss, even if several cryogens are used.

This application claims Paris Convention priority of DE 10 2004 037173.3 filed Jul. 30, 2004 the complete disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cooling device for re-liquefying cryogenicgases, comprising an outer jacket which delimits a vacuum chamber, and acryocooler cold head installed therein, which has at least two coldstages and is at least partially surrounded by a radiation shield.EP0905436, EP0905524, WO03036207, WO03036190, U.S. Pat. No. 5,966,944,U.S. Pat. No. 5,563,566, U.S. Pat. No. 5,613,367, U.S. Pat. No.5,782,095, U.S. Pat. No. 2002/000283, US2003/230089 e.g. describecooling of a superconducting magnet system with no or little cryogenloss using a cryocooler.

The e.g. two-stage cold head of the cryocooler is usually installed in aseparate sleeve assembly which is under vacuum (described e.g. in U.S.Pat. No. 5,613,367) or directly in the vacuum chamber of a cryostat(described e.g. in U.S. Pat. No. 5,563,566) such that the first coldstage of the cold head is rigidly connected to a radiation shield andthe second cold stage is connected in a heat-conducting manner to thehelium container either directly, or via a fixed thermal bridge, whereinthe helium container holds the superconducting magnet in liquid helium.Through recondensation of the helium, which evaporates due to externalheat input, on the cold contact surface in the helium container, theoverall heat input into the helium container can be compensated for,thereby providing operation with no or little cryogen loss of thesystem. In an alternative manner, the cold head can be inserted into aneck tube which connects the outer vacuum sleeve of the cryostat to thehelium container and is correspondingly filled with helium gas asdescribed e.g. in US2002/0002830A1. The first cold stage of thetwo-stage cold head is in fixed thermal contact with a radiation shieldand the second cold stage is freely suspended in the helium atmosphereto directly liquefy evaporating helium.

These variants have certain disadvantages. The design and constructionof the cryostat becomes more demanding and complex and installation ofthe additional sleeve which receives the cryocooler cold head generatesadditional heat input into the cold head. If an additional neck tube isused for the cold head, further heat is transferred into the heliumcontainer or cooler cold head due to thermal conduction in the heliumgas column and in the tube wall and through possible convection flow inthe helium gas. Moreover, fixed, rigid or even flexible thermal elementsconnected between the cold head and the cryostat can transfer cold headvibrations to the cryostat. Furthermore, in a temperature range below10K, magnetic regenerator materials are usually used in the regeneratorof the second stage of the cold head of cryocoolers such as pulse tubecoolers or Gifford-McMahon coolers, and the regenerator may berelatively close to the magnetic center of the NMR magnet system. Inconsequence thereof, the regenerator must generally be shielded toprevent disturbance of the magnetic field at the location of the NMRsample and to prevent the function of the regenerator from beingimpaired. Finally, an unstable state occurs when the cryocooler fails,and the temperatures of cryostat components such as e.g. the radiationshield continuously change until a new balanced state is reached. In amagnet system for high-resolution nuclear magnetic resonance (NMR)spectroscopy, this can preclude NMR measurements, since the shim stateof the magnet constantly changes or, in the worst case, the magnet runsdry and quenches.

One method for preventing some of these disadvantages while stillrealizing a partially loss-free cryogen system entails use of a devicecooled by a cryocooler which can be used for re-liquefying one singleevaporated cryogen. In a hitherto common cryostat arrangement for e.g. asuperconducting magnet system, the magnet is usually installed into acontainer filled with liquid helium at 4.2 K. The helium container isgenerally surrounded by a boil-off-gas-cooled radiation shield and afurther shield cooled with liquid nitrogen, such that the external heatinput onto the helium container is minimized. Due to passive cooling bythe evaporating cryogens, liquid helium and nitrogen must be refilled atcertain intervals.

JP11257770 and JP2000283578 propose inserting a heat-transferring devicein the form of a heat tube into the existing neck or suspension tubes ofa nitrogen container of a cryostat configuration, the heat tube beingconnected to the cold head of a cryocooler to re-liquefy evaporatingnitrogen (see also Advances of Cryogenic Engineering, 45, 41-48). Theliquefier is thereby directly flanged onto the cold head of theone-stage pulse tube cooler and consists of a thin tube in which thenitrogen vapor rises, is liquefied on a cold surface which is in contactwith the cold head, and runs downwards along the tube wall. This verythin tube is surrounded by a vacuum sleeve in its upper region and canbe directly inserted into a nitrogen neck tube or suspension tube toprevent or reduce evaporation of the nitrogen and nitrogen losses. Thehelium losses are not addressed, since only the nitrogen isre-liquefied.

In a similar manner, re-liquefaction of helium only has also beencarried out in a helium storage container using a two-stage cryocoolercold head.

In both cases (nitrogen or helium liquefier), the cold head of thecryocooler is in an outer jacket which delimits a vacuum chamber. Whenmulti-stage cryocoolers are used, parts of the cold head are usuallysurrounded by a radiation shield which is in contact with a cold stage(not the coldest stage) to ensure good insulation of the cold headagainst thermal radiation in the low temperature region.

As already mentioned above, different conventional cryostatconfigurations which are used, in particular, for magnet systems inhigh-resolution nuclear magnetic resonance (NMR) spectroscopy have morethan one cryogen. In addition to a container filled with liquid heliumand holding the magnet, an additional radiation shield is e.g. providedwhich is cooled with liquid nitrogen. In this manner, one would have touse a separate helium liquefier and a separate nitrogen liquefier toreduce both the helium and nitrogen losses or to obtain loss-freeoperation. This would considerably increase the number of devices, theinvestment, and the operating costs.

It is therefore the object of the present invention to provide a coolingdevice which, in an advantageous and straightforward manner, can beretrofitted to an existing cryostat configuration containing at leasttwo cryogenic liquids, in particular, a cryostat configuration whichcomprises a superconducting magnet arrangement, to eliminate or stronglyreduce loss of some or all existing cryogenic liquids relative toconventional devices.

SUMMARY OF THE INVENTION

Departing from prior art and in accordance with the invention, thisobject is achieved in that at least two cold stages of the cold head ofthe cryocooler are each individually connected in a heat-conductingmanner to a heat-transferring device which can be inserted into the neckor suspension tubes of a cryostat for keeping at least two differentcryogenic liquids.

A cooling device of this type offers the following advantages: Existingcryostat configurations, and in particular those which containsuperconducting magnets can be retrofitted without (or with only minor)adjustments, to permit operation with no or little cryogen lossrequiring only little extra hardware even if several cryogens are used.The cryostat must not be re-engineered. The additional heat input intothe cryostat produced by the device is small and can be predicted quiteprecisely when properly engineered. The heat-transferring devices inwhich the cryogens are liquefied are designed such that they can beintroduced in a contact-free manner into the neck or suspension tubes ofthe cryostat configuration. The evaporating gas is liquefied in athermodynamically effective manner, since the vapor is not overheatedand therefore need not be cooled down to the liquefying temperature. Thecryocooler cold head is placed at such a distance from the magneticcenter of a superconducting magnet arrangement in the cryostat thatdisturbances on the magnet arrangement caused by the magneticregenerator material are less severe than if the cold head were directlyintegrated into the cryostat. Conversely, the function of the cryocooleris also less impaired by the magnetic field of the magnet arrangement.If the cryocooler fails or must be switched off for maintenance work,the cryostat configuration still functions, e.g. for cooling asuperconducting magnet arrangement. This ensures high operationalreliability. Moreover, the user can freely select the mode of operation(conventional or without cryogen loss).

In a particularly preferred embodiment of the inventive cooling device,at least one of the heat-transferring devices has a cavity which isconnected to an open line, in particular a conduit. The cryogen whichevaporates from the liquid tank of the cryostat is guided through theconduit to the cavity at the cold stage, where it is liquefied. Thecondensed matter then flows back through the conduit into the liquidtank of the cryostat. This heat-transferring device functions likeconventional heat engineering heat pipes.

In a further preferred embodiment of the invention, at least one of theheat-transferring devices comprises a metallic connection with excellentheat transferring properties, at the end of which the cryogen evaporatedfrom the liquid tank of the cryostat is liquefied and subsequently flowsback into the liquid bath of the liquid tank of the cryostat. The otherend of this connection is flanged to a cold stage of the cold head ofthe cryocooler. Various combinations of the heat-transferring devicesare possible. A metallic connection with excellent heat conductingproperties may e.g. be flanged to the first cold stage of a two-stagecold head, while the second cold stage is connected to a conduit.

In particular for high-resolution NMR methods, the cryocooler isadvantageously a pulse tube cooler, since pulse tube coolers operatewith extremely low vibration. Moreover, pulse tube coolers also providereliable operation and require little maintenance.

The cooling device may also be operated with a Gifford McMahon cooler.One disadvantage of this cryocooler compared to a pulse tube cooler areincreased vibrations. This disadvantage can be overcome if soft sealingelements are provided between the cryocooler and the cryostatconfiguration, as is described below.

In a particularly advantageous manner, at least one connecting line,which is open at both sides, is provided to connect the cold head of thecryocooler to at least one neck or suspension tube of the liquid tankcontaining the cryogen of lowest-boiling temperature and into which noheat-transferring device is inserted, wherein the line is in thermalcontact with at least two cold stages of the cold head and may alsocontact a regenerator tube above the coldest cold stage, wherein theline terminates in the cavity mounted to the cold head after thermalcontact with the coldest cold stage, or is guided along the metallicconnection into the liquid tank. The gas in the line is cooled at thecold head of the cryocooler and liquefied at the coldest cold stage suchthat a flow is generated in the line through the neck or suspensiontubes to the cooling device due to the resulting suction. The gas flowcools the neck or suspension tubes thereby ideally completelycompensating for the heat input via the neck or suspension tubes. Thiscirculating flow for the cooling neck or suspension tubes furtherreduces heat input into the cryostat.

In a further development of this embodiment, a valve and/or a pump isprovided in the connecting line between the neck or suspension tubes andthe cold head to control the gas flow. The gas flow can be reduced orthe optimum gas flow can be adjusted as required if e.g. the suctioneffect at the cold head is so large that the gas flow becomes greaterthan required for optimum cooling of the suspension or neck tubes.

In an advantageous manner, helium can be liquefied at the coldest coldstage of the cold head at a temperature of 4.2 K or less to provide aplurality of possible applications in the region of low temperature. Thehelium loss and the refilling processes can be reduced or loss-freeoperation can be obtained if the cooling capacity of the cryocooler issufficiently large.

According to a further advantageous aspect, nitrogen can be liquefied at77 K or less at a cold stage of the cryocooler cold head. With the useof the heat-transferring devices in a cryostat configuration having acontainer with liquid nitrogen, the nitrogen loss can be reduced oreliminated during operation if the cooling capacity of the cryocooler issufficiently large.

In an advantageous embodiment, a cold stage of the cold head, which isnot the coldest, is connected in a heat-conducting manner to theradiation shield which, at least partially, surrounds the cold head. Inthis manner, the radiative heat input onto the colder components of thecold head is substantially reduced.

It is moreover advantageous if the heat-transferring device comes torest at least partially within the outer jacket of the cooling device,i.e. within the vacuum chamber. This is relevant in particular for thatpart of the heat-transferring device which is connected to the cold headof the cryocooler. This part of the heat-transferring device is therebyexcellently insulated against heat conduction towards the outside.

It is also very advantageous if the heat-transferring device issurrounded at least partially by a first tube in the region outside ofthe outer jacket. This tube thermally insulates the heat-transferringdevice. It may but must not have a constant diameter along its entirelength. It may be more favorable in view of construction to select thesmallest possible diameter for one part of the tube, and a largerdiameter for the rest.

In a preferred embodiment, the first tube which surrounds theheat-transferring device is open at one end, and that end is connectedto the vacuum chamber of the outer jacket, while the other end isconnected to the conduit or the metallic connection of theheat-transferring device in a gas-tight manner. If the vacuum chamber ofthe cooling device of this embodiment is evacuated, the part of theheat-transferring device surrounded by the first tube is also undervacuum. The heat-transferring device is then excellently insulated inthis region against thermal conduction towards the outside.

In another advantageous embodiment, the first tube surrounding theheat-transferring device is connected at both ends to the conduit or themetallic connection of the heat-transferring device in a gas-tightmanner, and evacuated via a separate connection. The interior of thetube can thereby be evacuated and the part of the heat-transferringdevice, which is surrounded by the tube can be excellently insulatedagainst thermal conduction towards the outside.

The conduit or the metallic connection of the heat-transferring devicecan advantageously at least partially surround a further second tubewhich is connected in a heat-conducting manner to the radiation shield.This tube is disposed within the first tube to provide vacuuminsulation, as described above. In this manner, the part of theheat-transferring device surrounded by the second tube is excellentlyinsulated against thermal radiation towards the outside.

With particular preference, the above-described tubes surrounding theheat-transferring device are flexible, at least in sections, and arepreferably designed as a bellows.

In a further favorable manner, the heat-transferring device is designedto be flexible, at least in sections, in particular as a bellows or inthe form of wires plaited into strands. In this embodiment of theinventive cooling device, the heat-transferring device and thesurrounding tubes are flexible to considerably facilitate installationthereof into the neck or suspension tubes of a cryostat configuration.

In this connection, it is also advantageous if the heat-transferringdevice and the surrounding tubes can be connected to and disconnectedfrom each other at at least one location using a gas-tight coupling. Thecoupling is designed such that the functionality of theheat-transferring device and the surrounding tubes is not impaired. Thissubstantially, facilitates mounting of the cooling device to a cryostatconfiguration.

In a further embodiment of the invention, the cooling device can bemounted to the cryostat for keeping cryogenic liquids either at theneck, at the suspension tubes, or on the outer jacket of the cryostatconfiguration.

In an alternative and preferred manner, the cooling device may bemounted outside of the cryostat e.g. on the room ceiling or on aseparate stand. In this case, the cryostat configuration does not haveto bear the weight of the cooling device. This can increase themechanical stability of the cryostat configuration.

In this connection, a soft connecting element which does not transmitvibrations is advantageously provided as a seal between the coolingdevice and the cryostat. This ensures that—in particular forhigh-resolution NMR methods—none or only little disturbing vibrations ofthe cooling device are transferred to the cryostat configuration.

Another possibility is mounting electric heaters to the cold stages ofthe cold head of the cryocooler. In case of surplus cooling capacity ofthe cryocooler, the heaters can be adjusted such that the cryocoolerexactly compensates for the heat input into the different containers ofthe cryostat configuration.

The advantages of the inventive cooling device are particularly wellutilized if they are part of a cryostat configuration.

In a particularly advantageous manner, the cooling device serves to coola superconducting magnet arrangement, in particular, a superconductingmagnet arrangement which is part of a nuclear magnetic resonanceapparatus, in particular, a magnetic resonance imaging (MRI) or nuclearmagnetic resonance spectroscopy (NMR) apparatus.

An electric heater can also be introduced into the liquid tank of acryostat configuration provided with the inventive cooling device via aneck or suspension tube of at least one liquid tank. In case of surpluscooling capacity of the cryocooler cold head which is integrated in thecooling device, the pressure in the liquid containers can thereby bekept at a constant level above the surrounding pressure. It is, however,also feasible to control the power of the cryocooler via its operatingfrequency and/or the fill amount of working gas in the cryocooler.

Further advantages of the invention can be extracted from thedescription and the drawings. The features mentioned above and below maybe used individually or collectively in arbitrary combination. Theembodiments shown and described are not to be understood as exhaustiveenumeration but have exemplary character for illustrating the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cryostat configuration with two liquid tanks forcryogenic liquids;

FIG. 2 a shows an inventive cooling device with heat-transferringdevices having a cavity;

FIG. 2 b shows an inventive cooling device with heat-transferringdevices comprising a metallic connection with excellentheat-transferring properties;

FIG. 3 shows a cooling device installed in a cryostat according to FIG.2 a;

FIG. 4 shows a cooling device according to the present invention whichis installed in a cryostat, with a connecting line which connects thecold head of the cryocooler to a suspension tube of a liquid tank;

FIG. 5 a shows a cooling device in accordance with the presentinvention, which is mounted on the cryostat;

FIG. 5 b shows a cooling device in accordance with the present inventionwhich is mounted to the room ceiling; and

FIG. 5 c shows a cooling device in accordance with the invention whichis mounted to a stand.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic illustration of a cryostat 1 with a magnetarrangement 5 as commonly used for MR applications. The cryostat 1comprises a liquid tank 2 a filled with helium which is connected viasuspension tubes 3 a to an outer jacket 4 of the cryostat 1 and containsa magnet arrangement 5. A further liquid tank 2 b is disposed about theliquid tank 2 a, which contains nitrogen and is connected via thesuspension tubes 3 b to the outer jacket 4 of the cryostat 1. The liquidtank 2 b with nitrogen is in thermal contact with the suspension tubes 3a. A boil-off-gas-cooled radiation shield 6 is disposed between the twoliquid tanks 2 a, 2 b and is also in thermal contact with the suspensiontubes 3 a.

FIG. 2 a shows an embodiment of an inventive cooling device 7. Thecooling device comprises an outer jacket 8 which delimits a vacuumchamber 9, and a cold head 10 of a cryocooler disposed therein whichcomprises at least two cold stages 11, 12 and is at least partiallysurrounded by a radiation shield 13. The cold stages 11, 12 of the coldhead 10 are each connected in a heat-conducting manner to aheat-transferring device 14 a, 14 b. The heat-transferring devices 14 a,14 b each have a cavity 15 a, 15 b, wherein each cavity 15 a, 15 b isconnected to a respective conduit 16 a, 16 b.

FIG. 2 b shows an alternative embodiment of the inventive cooling device7, wherein the heat-transferring devices 14 a, 14 b comprise connections17 a, 17 b with excellent heat conducting properties. These connectionsmay be e.g. in the form of cold fingers which are generally designed asmetal rods. Such a metal rod should have a maximum cross-sectionalsurface to ensure minimum temperature differences along the rod.

The conduits 16 a, 16 b can be inserted into the suspension tubes 3 a, 3b of the liquid tanks 2 a, 2 b of a cryostat 1. FIG. 3 shows aninventive cooling device 7 in the installed state. The conduits 16 a, 16b are located in the cryogen vapor above the liquid surface of thecryogens 18 a, 18 b disposed in the liquid tanks 2 a, 2 b. Theheat-transferring devices 14 a, 14 b are each connected in aheat-conducting manner to a cold stage 11, 12 of the cryocooler (FIGS. 2a, 2 b and 3). The cryogens 18 a, 18 b evaporated from the liquid tanks2 a, 2 b of the cryostat 1 are guided through the conduits 16 a, 16 binto the cavity 15 a, 15 b on the respective cold stage 12, 11 where thecryogens 18 a, 18 b are condensed and are thereby liquefied andsubsequently flow back through the conduits 16 a, 16 b into the liquidtanks 2 a, 2 b of the cryostat 1. The helium vapor can also be liquefiedat the end of a metallic connection 17 a, 17 b which is in contact withthe cold head 10 and has excellent heat conducting properties (FIG. 2b).

The cryogen 18 b with higher boiling temperature from the liquid tank 2b is thereby liquefied on the first cold stage 11 of the cold head 10while the cryogen 18 a with a lower boiling temperature is liquefied atthe second, colder cold stage 12 of the cold head 10. The invention alsocomprises cooling devices with a multi-stage cold head 10 such that, inprinciple, any number of cryogens, corresponding to the number of thecold stages of the cold head 10, can be liquefied.

The heat-transferring devices 14 a, 14 b are surrounded by a first tube19 a, 19 b to insulate them from thermal input, the first tube beingconnected to the vacuum chamber 9 of the outer jacket 8 of the coolingdevice 7 and can be evacuated together with the vacuum chamber 9 (FIGS.2 a, 2 b). To improve thermal insulation from the external heatradiation, a second tube 20 is disposed within the first tube 19 a whichis connected in a heat-conducting manner to the radiation shield 13. Thediameter of the first tube 19 b varies along its length in FIGS. 2 a, 2b and FIG. 3. It may be necessary to reduce the diameter of the tube atthe closed end such that it can be inserted into the suspension tube 3 bof the liquid tank 2 b in a contact-free manner. A bellows provides aflexible connection between the first tube 19 b and the outer jacket 8of the cooling device 7. A bellows may also be interposed between thefirst tube 19 a and the outer jacket 8 and in a section of the secondtube 20. The metallic connection 17 a, 17 b shown in FIG. 2 b can bemade flexible through flexible connecting elements 21 a, 21 b (such ase.g. wire plaited into strands). In case of surplus cooling capacity ofthe cryocooler, additional heaters (not shown) can be mounted to thecold stages 11, 12 of the cold head 10 of the cryocooler. Alternativelyor additionally, in case of surplus cooling capacity of the cryocooler,the pressure in the liquid tanks 2 a, 2 b for the cryogens 18 a, 18 bcan be kept constant using heaters 22 a, 22 b which are disposed in theliquid tanks 2 a, 2 b and which are e.g. inserted via remaining freeneck or suspension tubes 3 c, 3 d.

FIG. 4 shows an advantageous variant of the inventive cooling device,wherein a free neck or suspension tube 3 c of the cryostat 1 isconnected, via a line 23 which is open on both sides, after thermalcontact with the cold stages 11, 12 of the cold head 10, to the cavity15 a and therefore also to the liquid tank 2 a. A connection of thistype can also be realized with several free neck or suspension tubes 3c. The lines from the suspension tubes 3 c are initially combined intoone line 23. This line 23 is then guided through the outer jacket 8 ofthe cooling device 7 which contains the cold head 10 and is thermallycontacted using the heat exchanger 24 b, 24 a with at least two coldstages 11, 12 of the cold head 10 and possibly also with the regeneratortube 25 above the coldest cold stage 12 e.g. by wrapping it around theregenerator tube 25. After contact with the coldest cold stage 12, theline 23 terminates in the cavity 15 a mounted to the cold head 10 or isguided along the metallic connection 17 a into the liquid tank 2 a forthe cryogen 18 a (helium). The gas in the line 23 is cooled by the coldhead 10 and liquefied at the coldest cold stage 12, thereby generating aflow in the line 23 through the suspension tube 3 c towards the coolingdevice 7 due to the resulting suction. The heated gas flow cools thesuspension tube 3 c, whereby in the ideal case, the heat input iscompletely compensated or at least reduced via the suspension tube 3 c.The overall cooling capacity of the cryocooler slightly decreases due tothe additional load. The gain due to the reduced heat input is largerthan the loss in cooling power. In particular for systems with massiveneck or suspension tubes 3 c, a cryocooler with lower power can therebybe used. The heat-transferring devices 14 a, 14 b (heat tubes or coldfingers) may be made from two or more parts, which permits separationthereof using gas-tight couplings (not shown). This facilitatesinstallation and-disassembly. The line 23 has a valve 26 and a pump 27to control the gas flow through the line 23 and thereby adjust anoptimum gas flow. The line 23 may be provided with such a device (valve26 or pump 27) or such devices can be completely omitted. In theembodiment of FIG. 4 and also in the embodiment of FIG. 3, heaters 22 a,22 b are provided in the liquid tank 2 a, 2 b. For reasons of clarity,the connections are omitted in FIG. 4.

FIGS. 5 a through 5 c show various possibilities for fixing the coolingdevice 7. The vacuum container which contains the cold head 10 of thecryocooler can either be directly mounted on the outer jacket 4 of thecryostat 1 as shown in FIG. 5 a or externally e.g. on the room ceiling28 (FIG. 5 b) or on a separate-stand 29 (FIG. 5 c). A seal 30 must beused for mounting onto the cryostat 1. In case of external suspensiononly further sealing elements 31 a, 31 b are used between the vacuumchamber 9 and the outer jacket 4 of the cryostat 1 with the consequencethat no or only minimum vibrations of the cryocooler are transferred tothe cryostat 1. This is particularly favorable if the cooling device 7is used for cooling a cryostat configuration which contains asuperconducting magnet arrangement 5, in particular, if thesuperconducting magnet arrangement 5 is part of a nuclear magneticresonance apparatus, in particular magnetic resonance imaging (MRI) ormagnetic resonance spectroscopy (Nuclear Magnetic Resonance, NMR). Theinventive cooling device therefore allows high-resolution NMR methods.

In summary, a cooling device is provided which permits retrofitting toexisting cryostat configurations, and in particular such configurationswhich contain superconducting magnets without (or with only minor)adjustments to permit, in a straightforward manner, operation with no orlittle cryogen loss even if several cryogens are used.

LIST OF REFERENCE NUMERALS

-   1 cryostat-   2 a, 2 b liquid tanks-   3 a,b,c,d suspension tubes-   4 outer jacket-   5 magnet arrangement-   6 radiation shield of the cryostat-   7 cooling device-   8 outer jacket of the cooling device-   9 vacuum chamber-   10 cold head-   11 first cold stage-   12 second cold stage-   13 radiation shield of the cooling device-   14 a,b heat-transferring device-   15 a,b cavity-   16 a,b conduit-   17 a,b connection-   18 a,b cryogen-   19 a,b first tube-   20 second tube-   21 a,b connecting element-   22 a,b heater-   23 open line-   24 a,b heat exchanger-   25 regenerator tube-   26 valve-   27 pump-   28 room ceiling-   29 stand-   30 seal-   31 a,b sealing elements

1. A cooling device for re-liquefying cryogenic gases emanating from acryostat, the cryostat keeping at least two different cryogenic liquids,the cryostat having neck and suspension tubes, the cooling devicecomprising: a first cryocooler cold head stage; a second cryocooler coldhead stage; a radiation shield surrounding at least portions of saidfirst and said second stages; an outer jacket delimiting a vacuumchamber, said jacket surrounding said radiation shield and said firstand said second stages; a first heat transferring device inheat-conducting connection with said first cold head stage; and a secondheat transferring device in heat-conducting connection with said secondcold head stage, said first and said second heat transferring devicesstructured and dimensioned for insertion into the neck or the suspensiontubes of the cryostat.
 2. The cooling device of claim 1, wherein atleast one of said first and said second heat-transferring devices has acavity disposed in heat-conducting contact with said first or saidsecond cold stage, said cavity connected to an open line or conduitfeeding into a liquid tank of the cryostat, wherein cryogen isevaporated from the liquid tank and enters into said cavity where it isliquefied to subsequently flow back through said open line or conduitinto the liquid tank.
 3. The cooling device of claim 1, wherein at leastone of said first and said second heat-transferring devices has ametallic connection with excellent heat conducting properties at the endof which cryogen evaporated from a liquid tank of the cryostat isliquefied and subsequently returned into a liquid bath of the liquidtank.
 4. The cooling device of claim 1, wherein said cryocooler is apulse tube cooler.
 5. The cooling device of claim 1, wherein saidcryocooler is a Gifford-McMahon cooler.
 6. The cooling device of claim1, further comprising at least one connecting line, which is open atboth ends, to connect said cold head of said cryocooler to at least oneneck or suspension tube, in which no heat-transferring device isinserted, of a liquid tank containing a cryogen having a lowest boilingtemperature, wherein said line is in thermal contact with said first andsaid second cold stages of said cold head.
 7. The cooling device ofclaim 6, wherein said line is in thermal contact with a regenerator tubedisposed above a coldest cold stage and said line terminates in saidcavity or is guided along a metallic connection into the liquid tankafter thermal contact with said coldest cold stage.
 8. The coolingdevice of claim 6, further comprising a valve and/or a pump insertedinto said connecting line between the neck or suspension tube and saidcold head.
 9. The cooling device of claim 1, wherein helium can beliquefied at a temperature of 4.2 K or less at a coldest stage of saidcryocooler.
 10. The cooling device of claim 1, wherein liquid nitrogencan be generated at 77K or less at a cold stage of said cold head ofsaid cryocooler.
 11. The cooling device of claim 1, wherein a cold stageof said cold head of said cryocooler, which is not a coldest cold stage,is connected in a heat-conducting manner to said radiation shield whichat least partially surrounds said cold head.
 12. The cooling device ofclaim 1, wherein at least one of said first and said secondheat-transferring device is at least partially disposed within saidouter jacket.
 13. The cooling device of claim 2, wherein at least one ofsaid first and said second the heat-transferring device is at leastpartially surrounded by a first tube in a region outside of said outerjacket.
 14. The cooling device of claim 13, wherein said first tube isopen at one end, that end being connected to a vacuum chamber of saidouter jacket while an other end is connected in a gas-tight manner toone of said first and said second heat-transferring devices.
 15. Thecooling device of claim 13, wherein said first tube is connected at bothends in a gas-tight manner to one of said first and said secondheat-transferring devices and is provided with a separate connection forevacuation.
 16. The cooling device of claim 13, wherein said conduit ora metallic connection of said first or said second heat-transferringdevice is at least partially surrounded by a second tube which isconnected in a heat-conducting manner to said radiation shield, whereinsaid second tube is disposed within said first tube.
 17. The coolingdevice of claim 16, wherein said first and said second tubes areflexible or are designed as bellows.
 18. The cooling device of claim 2,wherein said conduit or a metallic connection comprises a flexiblesection, a bellows, or wires which are plaited into strands.
 19. Thecooling device of claim 16, wherein in at least one of said first andsaid second heat-transferring devices and a least one of said first andsaid second tubes can be connected and disconnected at at least onepoint using a gas-tight coupling.
 20. The cooling device of claim 1,wherein the cooling device can be mounted in a gas-tight manner to thecryostat for keeping cryogenic liquids.
 21. The cooling device of claim1, wherein the cooling device can be mounted outside of the cryostat.22. The cooling device of claim 21, further comprising a soft connectingelement which does not transmit vibrations and which is sealinglydisposed between the cooling device and the cryostat.
 23. The coolingdevice of claim 1, further comprising electric heaters mounted to atleast one of said first and said second cold stages of said cryocooler.24. A cryostat configuration characterized by the cooling device ofclaim
 1. 25. The cryostat configuration of claim 24, further comprisinga superconducting magnet arrangement, wherein said cooling device servesto cool said superconducting magnet arrangement.
 26. The cryostatconfiguration of claim 25, wherein said superconducting magnetarrangement is part of an apparatus for nuclear magnetic resonance,magnetic resonance imaging (MRI), or magnetic resonance spectroscopy(nuclear magnetic resonance NMR).
 27. The cryostat configuration ofclaim 24, further comprising an electric heater inserted into acryogenic liquid tank via the suspension or neck tubes thereof.